1. Field of the Invention
The present invention is related to the technology for controlling phase-locked loops.
2. Description of the Related Art
A phase-locked loop (PLL) is a control component widely used in the computers and communication circuits. The PLL can be used to trace the input frequency and output a frequency with the same phase of the input frequency.
Integer-N PLL, or Inter-N PLL, is a common phase-locked loop. FIG. 1 is a schematic diagram of an inter-n PLL. Generally, the phase-locked loop 100 at least comprises a voltage-controlled oscillator (VCO) 110 and a phase/frequency detector 120, and is coupled to a reference frequency source 130. The phase-locked loop 100 produces an output frequency fout synchronously according to a reference frequency fref provided by the reference frequency source 130. The voltage-controlled oscillator 110 can provide the output frequency fout to external circuits. The output frequency fout is fed back to the phase/frequency detector 120. The phase/frequency detector 120 is used to compare the reference frequency fref with the feedback output frequency fout. By monitoring the difference between the reference frequency fref and the output frequency fout, the output frequency and phase can be easily locked. In addition, the phase-locked loop 100 usually further comprises a charge pump 140 and a low-pass filter 150 to achieve better performance.
In the inter-N PLL 100, a frequency divider 160 is usually disposed between the reference frequency source 130 and the phase/frequency detector 120, and an additional frequency divider 170 disposed between the voltage-controlled oscillator 110 and the phase/frequency detector 120, as shown in FIG. 1. If the frequency divider 160 is an R-divider and the additional frequency divider is an N divider, the output frequency fout is N/R times the reference frequency fref (the phase-locked loop 100 amplifies the input frequency). Note that the greater the value of R of the divider 160, the smaller the resolution of the phase-locked loop 100. However, In order to maintain the same frequency variation, the value of N increases along with the increase of the value of R, and the phase noises of the entire system significantly increases as well. In response to the phase noises, a high-level low-pass filter 150 is thus required. The contemporary PLL also includes a fractional-N PLL. Although the fractional-N PLL can efficiently improve the frequency resolution without augmenting noise, it is more complicated in structure and thus expensive.
Therefore, the present invention provides a new phase-locked loop which has a simple structure and can be precisely controlled.